Bearings for electrical parts of automobiles such as an alternator, an electromagnetic clutch for a car air conditioner, an intermediate pulley and an electric fan motor, auxiliaries of engine and the like are used under severe environments such as high temperature, high speed, high load and vibration environment. In the bearings used in such severe environments, there is a problem recognized that spalling of the fixed ring surface of the bearing is caused in a period of time being as extremely short as 1/10 or less of a calculated service life theoretically estimated.
A major cause of such spalling is a structural change of steel due to hydrogen or the like which is considered to be generated by deterioration of grease used under severe environments or water in the grease and directly acts on the rolling contact surface (steel surface) between the rolling element and the fixed ring surface of the bearing. In addition, it is known that the hydrogen is generated by a decomposition of the grease or water in the grease due to static electricity caused by using the bearing.
In this connection, a grease for solving such a problem as mentioned above has been reported. For example, JP 3512183 B and JP 4102627 B disclose that antiwear property of the grease can be improved by using the grease comprising a predetermined extreme pressure additive, while improving conductivity of the grease is not considered.
Also, while JP 2007-046753 A and JP 2008-266424 A disclose a grease comprising a predetermined metal powder, it is not disclosed that conductivity of the grease is improved by defining the average particle diameter of the metal powder in a predetermined range.